Follow-up assessment of the microvascular function in patients with long COVID.

Long COVID is a complex pathophysiological condition. However, accumulating data suggests that COVID-19 is a systemic microvascular endothelial dysfunction with different clinical manifestations. In this study, a microvascular function was assessed in long COVID patients (n = 33) and healthy controls (n = 30) using flow-mediated skin fluorescence technique (FMSF), based on measurements of nicotinamide adenine dinucleotide fluorescence intensity during brachial artery occlusion (ischemic response, IR) and immediately after occlusion (hyperemic response, HR). Microcirculatory function readings were taken twice, 3 months apart. In addition, we quantified biochemical markers such as the serum L-arginine derivatives and hypoxia-inducible factor 1α (HIF1α) to assess their relation with microvascular parameters evaluated in vivo. In patients with long COVID, serum HIF1α was significantly correlated to IR (r = -0.375, p < 0.05). Similarly, there was a significant inverse correlation of serum asymmetric dimethyl-L-arginine levels to both HR (r = -0.343, p < 0.05) and HR (r = -0.335, p < 0.05). The IR parameters were found lower or negative in long COVID patients and recovered in three-month follow-up. Hypoxia sensitivity value was significantly higher in long COVID patients examined after three months of treatment based on the combination of ACE-inhibitors and beta-adrenolytic compared to baseline condition (85.2 ± 73.8 vs. 39.9 ± 51.7 respectively, p = 0.009). This study provides evidence that FMSF is a sensitive, non-invasive technique to track changes in microvascular function that was impaired in long COVID and recovered after 3 months, especially in patients receiving a cardioprotective therapy.